GEOGRAPHICAL EVOLUTION. 601 



A curious fact deserves to be noticed here. During the convul- 

 sions by which the sediments of the Silurian sea-floor were crumpled 

 up, crystallized, and elevated into land, the area of Russia seems to 

 have remained nearly unaffected. Not only so, but the same immu- 

 nity from violent disturbance has prevailed over that vast territory 

 during all subsequent geological periods. The Ural Mountains on the 

 east have again and again served as lines of relief, and have been from 

 time to time ridged up anew. The German domains on the west have 

 likewise suffered extreme convulsion. But the wide intervening pla- 

 teau of Russia has apparently always maintained its flatness either as 

 sea-bottom or as terrestrial plains. 



By the time of the coal-growths, the aspect of the European area 

 had still further changed. It then consisted of a series of low ridges 

 or islands in the midst of a shallow sea or of wide salt-water lagoons. 

 A group of islands occupied the site of some of the existing high 

 grounds of Britain. A long, irregular ridge ran across what is now 

 France from Brittany to the Mediterranean. The Spanish Peninsula 

 stood as a detached island. The future Alps rose as a long, low ridge, 

 to the north of the eastern edge of which lay another insular sj:>ace, 

 where now we find the high grounds of Bavaria and Bohemia. The 

 shallow waters which wound among these scattered patches of land 

 were gradually silted up. Many of them became marshes, crowded 

 with a most luxuriant cryptogamic vegetation, specially of lycopods 

 and ferns, while the dry grounds waved green with coniferous trees. 

 By a slow intermittent subsidence, islet after islet sank beneath the 

 verdant swamps. Each fresh depression submerged the rank jungles 

 and buried them under sand and mud, where they were eventually 

 compressed into coal. To this united cooperation of dense vegetable 

 growth, accumulation of sediment, and slow subterranean movement, 

 Europe owes her coal-fields. 



All this time the chief area of high ground in Europe appears still 

 to have lain to the north and northwest. The old gnarled gneiss of 

 that region, though constantly worn down and furnishing materials 

 toward each new formation, yet rose up as land. It no doubt re- 

 ceived successive elevations, during the periods of disturbance, which 

 more or less compensated for the constant loss from its surface. 



The next scene we may contemplate brings before us a series of salt 

 lakes, covering the center of the continent from the north of Ireland 

 to the heart of Poland. These basins were formed by the gradual 

 cutting off of portions of the sea which had spread over the region. 

 Their waters were red and bitter, and singularly unfavorable to life. 

 On the low intervening ridges a coniferous and cycadaceous vegeta- 

 tion grew, sometimes in quantity sufficient to supply materials for the 

 formation of coal-seams. The largest of these salt lakes stretched 

 from the edge of the old plateau of Central France along the base of 

 the Alpine ridge to the high grounds of Bohemia, and included the 



